Chapter 5 Dike Patterns in Diapirs beneath Oceanic Ridges: The Oman Ophiolite

Abstract

This chapter explores the problem of magma-transport structures and diking at oceanic ridges by presenting a structural study of dike orientations in the peridotite section of the Oman ophiolite. Dike orientation was systematically measured in the vicinity of mantle diapirs exposed in the Oman ophiolite. Collectively, diking reflects a continuous history of melt injection, from the early gabbro and pyroxenite dikes emplaced in a concurrently melting peridotite to the dominantly gabbro dikes emplaced in peridotites at subsolidus temperatures and to the microgabbro and diabase dikes emplaced in peridotites that have cooled below 500°C. The diabase dikes—emplaced when the ridge was cooling—have excellent preferred orientations that are mainly parallel to the sheeted-dike trend. A group of dikes is oriented parallel to the sheeted-dike complex and ascribed to a relatively regionally organized lithospheric stress field. Another group is composed of flat-lying sills and moderately inclined dikes controlled by the locally organized asthenospheric stress field created by the diapir flow divergence just below the ridge. Melt is injected in either the sills or the dikes. This feature suggests a relatively rapid shift in the orientation of the local stress field, which is attributed to the effects of a large-volume and relatively violent melt surge from the rising diapir into the newly accreting crust. Because orientations in the high-temperature dikes are, at present, so poorly clustered, more work is required before conclusions on this interesting class of intrusions may be drawn.